Before I devised my invention, rope implements used to handle bulk commodities, ranging from coal and iron ore to scrap metal and to processed foodstuffs like soybean meal, have suffered from inefficiencies caused by a high ratio of implement weight to load weight. For example, present clamshell buckets used to unload ore-carrying ships can pick up at the most only 1 to 11/2 pounds of ore per pound of bucket weight. Since the unloading capacity of an ore terminal is limited by the number of times per day that a bucket can be lowered into the hold of a ship and raised from it, terminal operators may achieve substantial savings, which can be passed on to the shippers and ultimately to the consumers of the product unloaded, by increasing the weight of material unloaded with each deployment of the bucket.
With this in mind, I set to inventing improvements in bucket design, which are equally applicable to grapples, hook blocks and all other implements using rope-and-pulley arrangements to lift loads. The economic incentive is considerable. At the wage, equipment charge and demurrage rates prevailing in August 1978, the cost of buckets, conveying equipment, stevedoring and demurrage average about $40,000 per day at the ore terminals in Burnside and New Orleans, La. Thus, a 50 percent increase in production will save approximately $13,000 per day.
Typical prior art buckets are depicted in U.S. Pat. Nos. 197,384, to Lord, 515,117, to Curtis et al., and 1,956,079 to Neffendorf et al. The Lord and Curtis buckets, which typify the clamshell buckets commercially available in the United States for the past 80 years or more, each have ropes or chains reeved about a large sheave which is concentrically located about the center shaft from which the bucket halves or scoops are hinged. This large sheave, which usually is a solid casting and may itself weigh a ton or more in larger buckets, contributes a high proportion of the total weight of the bucket. The Curtis bucket possesses the additional weight disadvantage of employing a large unitary sheave on the head assembly of the bucket. The Neffendorf bucket uses additional sheaves, called axles, which are affixed to the bucket hinges. The wire rope is reeved around these axles to provide greater closing force. In practical application, however, the added weight of these auxiliary axles in conjunction with the existing weight of the large head sheave, not withstanding the lack of a unitary center pivot assembly as described below, fails to improve the bucket to load weight ratio over the more conventional Lord and Curtis buckets.
Many of these prior art buckets mount the large central sheave between stubbed shaft ends, which are welded or otherwise connected to the sheave.
The bucket industry in the United States has innovated little since the advent of the basic Lord and Curtis designs. In the United Kingdom, however, bucket manufacturers have long been interested in improving bucket efficiency. In the 1949 Proceedings of the Institution of Mechanical Engineers, at pages 154 through 172, G. T. Shoosmith discusses the efforts of William Cory and Son, Ltd., in modernizing a coal yard. He emphasizes the understanding in this art, which prevails today, that a large sheave to rope diameter ratio is necessary to provide maximum bucket weight capacity during high-speed operation. The buckets depicted in the article put this understanding into practice by using large unitary sheaves both in the bucket head and center pivot assemblies. H. N. Wilkingson, of the British Iron and Steel Research Association, summarized the results of ten years of bucket design research in an article appearing at pages 831 through 846 of the 1963-64 Proceedings of the Institution of Mechanical Engineers. He dismissed lever-arm clamshell buckets as lacking interest and concluded that "wide-span" buckets were superior to clamshell buckets in efficiency, weight for weight. His article does not, however, address the technique of improving the lift ratio of buckets by reducing bucket weight. Mr. Wilkinson noted that efficiency could be improved by increasing the reeving, but doubted "whether in practice space could be found for the additional sheaves" (p. 839). Additional reeving also increases bucket cycle time, which is disadvantageous. His tests showed (p. 840) that the "sheaves should be as far from the pivot point as practicable, thereby increasing the closing torque, but not so far that grab [bucket] capacity is reduced owing to their taking up space in the shells." The wide sheaves referred to by Mr. Wilkinson correspond exactly to the "axles" of the Neffendorf bucket.